What is the difference between CCT and CRI?

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Before energy-efficient lighting became mainstream, choosing a light bulb was quite straightforward. The 40-watt bulb not giving you enough light? Choose a 60-watt and get more light. Simple and easy!

LED technology has allowed for a wide range of possibilities in terms of performance and features, and while this might be seen as a step forward in terms of environmental technologies and for industrial or commercial applications, this makes things much more complicated for the average consumer who just wants to purchase a few light bulbs.

With all of the color and light terms, it can be overwhelming and frustrating to know what kind of bulb to purchase. We're here today to help explain and distinguish between two of the more challenging terms that are often confused with each other - CCT (correlated color temperature) and CRI (color rendering index).

What is CCT, or Correlated Color Temperature?

CCT is a number, measured in degrees Kelvin, that helps to describe the relative warmth or coolness of a light source. Most light bulbs will range from 2700K (warm, incandescent color) to 5000K or higher (crisp, white daylight color).

For most residential applications, 2700K and 3000K are preferred color temperature options as they both create a nice, warm appearance that creates an inviting and relaxing environment.

For retail or commercial applications, 4000K is a popular color option as it provides for a crisper, more energetic shade of white. For industrial or task-oriented applications, 5000K or even 6500K is a preferred color option, as these color temperatures best match natural daylight.

What is CRI, or Color Rendering Index?

The CRI value of a light source describes how accurately a light source illuminates colors of an object. It is scored on a scale with 100 being the best. Most standard bulbs have a CRI value of 80, while high CRI bulbs have a CRI value of 90 or higher.

For example, a painting shown under a 70 CRI light source may exhibit colors that don't appear correct, or accurate. When seen under a 95 CRI light source, however, the colors of the painting will likely look accurate and natural.

The difference between CRI and CCT explained

As explained above, CCT and CRI measure two different aspects of color. CCT tells us the color of the light emitted by the light bulb, and is immediately visible to the casual observer by looking directly at the light source.

On the other hand, the CRI value does not tell us the color of the light itself. Rather, it tells us about the color appearance of objects under the light source (the light source "renders" the colors of an object, hence the term). You cannot determine a light bulb's CRI value by looking at the light itself. Instead, a light bulb's CRI can only be estimated to the naked eye by looking at the colors of an object illuminated by the light bulb. An illustrative example of this principle in action is when photographers and artists use a "color checker," which uses a palette of standardized colors to estimate the color rendering quality.

The only way to measure a light source's CRI value is by using specialized spectral measurement devices. Lighting manufacturers rely on data from these devices to publish and guarantee color rendering related metrics.

The relationship between CCT and CRI

Although CCT and CRI describe two different aspects of light color, they are nonetheless intimately related in their calculations. As mentioned above, CRI can be thought of as determining a light source's accuracy. In determining "accurate," however, we must first determine what the reference point for this "accuracy" should be. For example, when judging the color appearance of a painting, how do we know what it should look like? In other words, what is the reference standard?

When we refer to light accuracy, we must first determine the color temperature of a light source, so that we have a proper reference point. Each color temperature has a "reference standard" which is considered to be the light source that provides the most accurate, or natural lighting. For example, 2700K has a reference standard that is approximately equal to an incandescent light bulb. 6500K on the other hand, has a reference standard that is approximately equal to natural daylight (at noon on a midsummer day at the equator).

To continue with the painting example, if we have a light bulb whose color temperature we measure to be 2700K, we would judge the appearance of colors as compared to an incandescent bulb. We would not compare them to how they appear to natural daylight, because the color temperature of 2700K indicates that this is a warm, yellowish light source that does not come close to natural daylight.

On the other hand, if we have a light bulb whose color temperature is measured to be 6500K, we would compare the color appearance to its color appearance under natural daylight.

Why CCT is more fundamental and important than CRI

The explanation above should have made clear that CRI requires a color temperature value in order to determine what we are comparing color appearance against.

CRI is certainly an important metric that helps explain color quality, but it is almost meaningless when used alone without a color temperature value. Given a light bulb's 95 CRI rating, you might be impressed and conclude that it must be very accurate. But accurate when compared to what? Incandescent bulb light color (2700K), natural sunlight (5000K) or natural daylight (6500K)?

Think first about the color temperature requirements for your application, and then worry about CRI after. Are you looking to replicate the light of natural daylight? Pick a high color temperature value (5000K or higher), and then CRI value next. A 2700K light bulb with 95 CRI, even with a high CRI rating, will not come even close to replicating natural daylight due to its color temperature being way off.

Now, in your quest to replicate natural daylight, let's say you find a 6500K bulb but with a low CRI. In this case, the color of light emitted by the bulb might look the same as natural daylight (due to the color temperature value), but as soon as the light lands on any surface with color, you will find that the colors do not appear the same as under natural daylight (due to the low CRI value).

Next steps?

We hope this article helped to clarify any confusion between CCT and CRI values of a light source. Have you determined the color temperature needs for your application? If not, we recommend making a decision on this first. Once decided, determine if color accuracy is important for you and set appropriate requirements on the CRI value as needed. Waveform Lighting specializes in high CRI LED lighting products, and we're confident you'll find something that fits your high CRI needs.

Still not sure how CCT and CRI are related? We have an interactive graphic available here that illustrates the difference between CCT and CRI.

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